Saturday, October 2, 2010

Abstraction in Java or Object oriented programming is a way to segregate implementation from interface and one of the five fundamentals along with Encapsulation, Inheritance, Polymorphism, Class and Object. Abstraction in Java is achieved by using interface and abstract class in Java. An interface or abstract class is something which is not concrete , something which is incomplete. In order to use interface or abstract class we need to extend and implement abstract method with concrete behavior. One example of Abstraction is creating interface to denote common behavior without specifying any details about how that behavior works e.g. You create an interface called Server which has start()and stop() method. This is called abstraction of Server because every server should have way to start and stop and details may differ. As I said earlier Abstraction in Java is implemented using abstract class and interface as discussed in next section. In fact what is abstraction in Java, difference between Abstraction and Encapsulation is also a very popular core Java interview because strong OOPS skill is one of the primary requirement for Java developers.

What is abstract class in Java

An abstract class is something which is incomplete and you can not create instance of abstract class. If you want to use it you need to make it complete or concrete by extending it. A class is called concrete if it does not contain any abstract method and implements all abstract method inherited from abstract class or interface it has implemented or extended.By the way Java has concept of abstract classes, abstract method but a variable can not be abstract in Java. Popular example of abstract class in Java is ActionListener which has abstract method called actionPerformed(ActionEvent ae). This method is called when an ActionEvent is fired like when you click on JButton. Its common in java to attach ActionListener with JButton by implementing abstract method actionPerformed(ActionEvent ae) using Anonymous class, as shown in below Example :

An abstract method in Java doesn't have body , its just a declaration. In order to use abstract method you need to override that method in sub class.

so when do you use abstraction ? ( most important in my view )

when you know something needs to be there but not sure how exactly it should look like. e.g. when I am creating a class called Vehicle, I know there should be methods like start() and stop() but don't know how that start and stop method should work, because every vehicle can have different start and stop mechanism e..g some can be started by kicking or some can be by pressing buttons . Same concept apply to interface in Java as well, which we will discuss in some other post.

So implementation of those start() and stop() methods should be left to there concrete implementation e.g. Scooter , MotorBike , Car etc.

Abstraction Using Interface in Java

InJava Interface is an another way of providing abstraction, Interfaces are by default abstract and only contains public, static, final constant or abstract methods. Its very common interview question is that where should we use abstract class and where should we use Java Interfaces in my view this is important to understand to design better Java application, you can go for java interface if you only know the name of methods your class should have e.g. for Server it should have start() and stop() method but we don't know how exactly these start and stop method will work. if you know some of the behavior while designing class and that would remain common across all sub classes add that into abstract class. Interface like Runnable interface is good example of abstraction in Java which is used to abstract task executed by multiple thread. Callable is another good abstract of a task which can return value.

Abstraction : Things to Remember

1) Use abstraction if you know something needs to be in class but implementation of that varies. Abstraction is actually result of thought process and it really need good experience of both domain and Object oriented analysis and design to come up with good abstraction for your project.

2) In Java you can not create instance of abstract class using new operator, its compiler error. Though abstract class can have constructor.

3) abstract is a keyword in Java, which can be used with both class and method. Abstract class can contain both abstract and concrete method. Abstract method doesn't have body, just declaration.

4) A class automatically becomes abstract class when any of its method declared as abstract.

5) abstract method doesn't have method body.

6) In Java, variable can not be made abstract , its only class or methods which would be abstract.

7) If a class extends an abstract class or interface it has to provide implementation to all its abstract method to be a concrete class. alternatively this class can also be abstract.

@siddu, major difference between abstract class and interface in Java is that, Interface is complete abstract while Abstract Class can contain non-abstract method.Also everything in interface by default public.In terms of using, interface is generally used to represent behavior e.g. Serializable, Runnable, Clonnable and in Java one Class can extend only one class but multiple interface so it make sense to use interface for behavior than using abstract class in Java.

@Anonymous, There is nothing like abstract data type in my knowledge, though you have abstract class in Java which you may refer abstract data type. What is important is that you can not create instance of abstract class in Java, they are incomplete and require to be implemented by sub classes. Abstract classes can or can not contain abstract methods but if you have any abstract method that class automatically becomes abstract in Java.

Abstraction arises from a reorganization of similarities between certain objects, situations, or processes in the real world, and decision to concentrate upon these similarities and to ignore for the time being the differences.

Abstraction is a subjective term and encapsulation is objective, like both are the two different view of seeing the one thing. Abstraction says what a object can do, but encapsulation say how that object can do that.

Abstraction is much more than abstract class and interface. Abstraction is key to design pluggable systems where you can easily replace parts with better ones. Abstraction is closely related to Specification and until your class meets a specification can plug it into System. For example MOUSE in your computer is not tied to any particular company and can be replaced by any company. Abstraction in Java or any Object oriented language is key to design loosely coupled applications.

Abstraction: showing functionality hiding the implementation which is abtsract, abstraction is showing functionality irrelevent to its implementation in which whose implementation can change overtime with different type of instances but purpose is retained. doing this complexity of its implementation is hidden.

To be honest I never understood abstract. (I'm in high school) I mean, why not just make a concrete one and simply not implement it? That way if for some reason you had to implement it you could, and if you don't, no harm done. Can I get an explanation?﻿

@Liam, abstraction is really nice way to make your programming simpler. It allows you to deal with things which doesn't exists in reality but they are simpler to deal with. A good example of abstration is Queue data structure, which allows two process to share data e.g. in producer consumer problem. This queue can be impelmented using sockets, pipes, or shared memory, but your program don't need to worry about. you also don't need to change your program if underlying implementation of Queue changes. In short, Abstraction make your program more flexible and simpler.

When you have written, that "you can not create instance of abstract class" why there is "new ActionListener()" which is interface based on http://docs.oracle.com/javase/7/docs/api/java/awt/event/ActionListener.html